Hydraulic systems employing hydraulic oil as the working fluid are used extensively. Additives having zinc and/or phosphorous have been used in such hydraulic oils to increase operational properties including thermal stability, oxidation stability, anti-wear properties, and the like. One especially preferred additive in hydraulic oils or fluids is zinc dialkyldithiophosphate (ZDTP). Zinc dialkyldithiophosphate can be represented by the general structure: ##STR1## where R is an alkyl group such as, for example, octyl. These zinc dialkyldithiophosphates are viscous, sticky materials which are soluble in oil and insoluble in water. In high-zinc-type hydraulic fluids, the level of added zinc dialkyldithiophosphate is over one weight percent. In such fluids, oil sludge formation is a significant problem, especially in high pressure hydraulic systems having close tolerance parts. Low-zinc-type hydraulic fluids, having less than about 0.7 weight percent zinc dialkyldithiophosphate (generally in the range of about 0.3 to 0.5 weight percent) were developed, in part, to reduce the tendency of sludge formation. Nonetheless, sludge formation has remained a significant problem with hydraulic oils containing zinc dialkyldithiophosphate or other zinc- and phosphorous-containing additives.
Sludge formation in hydraulic oils results in a reduction in the useful life of the hydraulic oil as well as costly downtime. Such downtime can be significant, especially if the sludge clogs filters or deposits on internal surfaces of the hydraulic system.
Generally such sludge formation is not thought to be cause by deterioration of the hydraulic oil components themselves. Rather, most of the operational problems appear to result from contamination from outside the hydraulic system and the interaction of such contamination with the components of the hydraulic oil. One significant mechanism for sludge formation appears to be the reaction of aqueous contaminates with additives containing zinc and/or phosphorous. In many instances, the sludge recovered from hydraulic systems is rich in zinc- and phosphorous-containing compounds.
Hydraulic oils leaking into other systems may also clause similar problems. For example, cutting or machine tools normally employ aqueous based cutting fluids. Hydraulic oil which leaks into such cutting fluid systems can significantly reduce the useful lifetime of the cutting fluid. When the hydraulic oil leaks into the cutting fluid, the zinc dialkyldithiophosphate or other zinc- and phosphorous-containing additives can be decomposed by reaction with water to form zinc polyphosphates, water soluble phosphates, and alkyl sulphides. The phosphates appear to concentrate in the water phase of the cutting fluid and eventually to destablize the cutting fluid emulsion by forming a "sticky cream" containing metallic fines and unemulsified oil. The "sticky cream," in addition to reducing the effectiveness of the cutting fluid, can build up on machines, tools, and parts produced.
Attempts have been made to reduce the tendency of hydraulic oils containing zinc and phosphorous compounds (especially zinc dialkyldithiophosphates) to form sludges during use in a hydraulic system or when the hydraulic oil contaminates other systems. Generally, simply eliminating the zinc and phosphorous compounds from the hydraulic oil formulation is not satisfactory because of a reduction in anti-wear and other properties. Others have attempted to extend the lifetime of such fluids by removal of decomposition products using chemical treatment and vacuum filtration techniques. Such "cleaning" processes are more useful in treating fluids which become contaminated with hydraulic oils since the hydraulic oils, and thus the contaminated products, form only a relatively small proportion of the fluid to be treated. Others have used dispersants in zinc-containing hydraulic oils to help disperse the sludge formed from the zinc dialkyldithiophosphates. Such dispersants may even delay the formation of the sludge.
None of the methods of the prior art have been completely successful in providing a hydraulic oil with acceptable operational properties while minimizing the sludge formation of the hydraulic oil. It is desirable, therefore, to provide an essentially zinc-free and phosphorous-free hydraulic oil with good operational properties and minimal sludge formation properties. The present invention provides such a hydraulic oil.